In a liquid crystal display (LCD) device, the application of a voltage changes the molecular orientation of the liquid crystal fluid, and the resulting change in optical characteristics such as double refraction, optical rotation, dichroism, or optical scattering, caused by the reorientation, is converted to a visible change. In other words, the LCD is a passive display device that utilizes the modulation of light within a liquid crystal cell. The liquid crystal cell is constructed with a layer of liquid crystal about 10 .mu.m thick sandwiched between two glass substrates on which are formed transparent electrodes, and a molecular orientation film is formed on the surface of the electrode to impart a fixed orientation to the liquid crystal molecules.
The most widely used type of LCD is the nematic-type display (sometimes called twisted nematic or TN), in which the liquid crystal molecular orientation is twisted about 90.degree. between the electrodes. This twisted molecular orientation disappears under the application of a voltage, and by putting the liquid crystal cell between crossed polarizing elements, bright and dark appearances can be obtained by applying or removing the voltage. This switching in normal TN or STN displays usually has a transition time on the order of milliseconds to hundreds of milliseconds. The liquid crystals used in TN displays are principally biphenyl, phenylcyclohexane and cyclohexylcarboxylic esters nematic liquid crystals. Nematic type LCD's operate at typical voltages between 1-5 volts, and have become quite popular, finding many uses in industrial, military and consumer products.
Another type of LCD uses cholesteric liquid crystals (e.g. fatty acid esters of cholesterol) to form a bistable device. A bistable optical device is one which can exist in either of two stable states of optical transmission. As in the TN display, the image is formed of bright and dark areas that are created by applying a voltage to the electrodes. However, unlike the TN display, the image does not disappear when the electrical field is removed, but persists for a period of time. The cholesteric display is thus said to exhibit bi-stability, which means that when a pixel is pulsed with a voltage, the pixel will stay "on", even after the voltage is turned off, until the pixel is turned off by a pulse of a different (typically lower) voltage. One disadvantage of this type of display is that it only operates at high voltages, usually in the range of 30-40 volts. This type of display has found very limited application, because the image formed is not easily removed, thus limiting its usefulness.
These two types of devices can roughly be classified into nematictype devices and bistable-type devices. It might be advantageous if one were able to combine the desirable features of the TN display with the desirable features of the bistable display.